I have a long header on my document (WORD 2010) and I want to remove it from the two first pages. But I don't know how to do that.
Headers and footers in Word are related to sections in a document.
In order to have different headers/footers on different pages (or in this case, a header on some pages but no others), they must be in different sections.
You can insert a section break by selecting Page Layout > Page Setup > Breaks, from the ribbon.
Once you have inserted a section break, when you look at headers/footers you will see they have a section associated with them, simply delete the header out of section one and leave it in for section two.
You can change the header settings
A document must first be divided into sections in order to create a different header or footer for part of a document.
If you haven't done so already, insert a section break where you want to start a new section that contains a different header or footer.
Click in the section for which you want to create a different header or footer.
Microsoft Word no longer displays "Same as Previous" in the upper-right corner of the header or footer.
Change the existing header or footer, or create a new one for this section.